$(document).ready(function(){	
	$('div#top_menu ul.top_menu li').hover(function() {
		//Find a child of 'this' with a class of .sub_nav and make the beauty fadeIn.
		$(this).find('.sub_nav').show();
	}, function() {
		//Do the same again, only fadeOut this time.
		$(this).find('.sub_nav').hide();
	});	
});